1. Field of the Invention
The present invention relates to surface acoustic wave resonators including quartz substrates. In particular, the present invention relates to a surface acoustic wave resonator in which a SiO2 film covers an IDT electrode, and a surface acoustic wave device including such a surface acoustic wave resonator.
2. Description of the Related Art
Surface acoustic wave devices are widely used in cellular phones and in vehicle-mountable electronic appliances. A surface acoustic wave device includes a piezoelectric substrate and an IDT electrode provided on the piezoelectric substrate. Various piezoelectric single crystals and piezoelectric ceramics are used as the materials for the piezoelectric substrate.
Japanese Unexamined Patent Application Publication No. 2006-165746 describes a surface acoustic wave device, which has an IDT electrode made of Al or an alloy primarily including Al, disposed on a quartz substrate. FIG. 11 is a cross-sectional view schematically showing a surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2006-165746.
A surface acoustic wave device 101 includes a packaging member 102 having an opening that opens upward. A surface acoustic wave resonator 103 is disposed in the opening. In other words, the surface acoustic wave resonator 103 is mounted on a bottom surface inside the opening of the packaging member 102.
The surface acoustic wave resonator 103 has a rotated Y-cut quartz substrate 104 which has a cut angle θ of −64.0°<θ<−49.3°. An IDT electrode 105 and reflectors 106 and 107 arranged on two sides of the IDT electrode 105 in the surface acoustic wave propagation direction are provided on the quartz substrate 104. The IDT electrode 105 is arranged so that the surface acoustic wave propagation direction is 90° with respect to the X axis. The IDT electrode 105 and the reflectors 106 and 107 are made of aluminum and have a thickness of 0.04λ to 0.12λ, where λ is the wavelength of the surface acoustic waves. The duty ratio of the IDT electrode 105 is set to 0.5 to 0.65.
The opening in the surface acoustic wave device 101 in which the surface acoustic wave resonator 103 is disposed is covered with a cover member not shown in the drawing to hermetically seal the surface acoustic wave resonator 103.
The surface acoustic wave device 101 can have a lower capacity ratio to enhance frequency controllability by setting the cut angle θ of the quartz substrate 104 within the above-described specific range and by adjusting the thickness and the duty ratio of the IDT electrode within the above-described specific ranges.
Japanese Unexamined Patent Application Publication No. 2006-165746 also discloses that a protective film made of a SiO2 film or a protective film made of anodized aluminum may be arranged to cover the IDT electrode.
Japanese Unexamined Patent Application Publication No. 2002-135077 discloses that aluminum and alloys primarily including aluminum are widely used as materials for IDT electrodes of surface acoustic wave devices. Japanese Unexamined Patent Application Publication No. 2002-135077 describes problems that arise in this type of surface acoustic wave devices. The problems include the generation of a transverse mode caused by the IDT defining a waveguide and the generation of ripples caused by the transverse mode in the pass band. The invention disclosed in Japanese Unexamined Patent Application Publication No. 2002-135077 provides a structure that can reduce the transverse mode ripple. In particular, an IDT electrode defined by an electrode having a larger load effect than aluminum is provided on a quartz substrate, and the metallization ratio d and the normalized thickness h/λ of the IDT are controlled to reduce the transverse mode ripple to 0.5 dB or less, where the metallization ratio of the IDT electrode is represented by d and the electrode thickness is represented by h. In other words, Japanese Unexamined Patent Application Publication No. 2002-135077 discloses that the transverse mode ripple can be suppressed by providing an IDT electrode that is made of an electrode material, such as tantalum, that has a mass greater than that of aluminum on a quartz substrate and by adjusting the metallization ratio d and the electrode thickness h of the IDT electrode in the specific ranges.
Japanese Unexamined Patent Application Publication No. 2006-165746 describes that the surface acoustic wave device 101 may include a protective film made of SiO2 or anodized aluminum to protect the IDT electrode. However, it has been found that, with respect to the surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2006-165746, if a SiO2 protective film covering the IDT electrode is actually provided on a quartz substrate, large spurious responses are generated between the resonance frequency and anti-resonance frequency.
In a surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2002-135077, an IDT electrode is made of an electrode material having a mass load effect that is larger than that of aluminum and the metallization ratio d of the IDT electrode and the normalized thickness of the IDT electrode are adjusted within specific ranges, as described above, to suppress the transverse mode spurious responses. However, Japanese Unexamined Patent Application Publication No. 2002-135077 does not disclose the formation of a protective film made of SiO2. In other words, the configuration described in Japanese Unexamined Patent Application Publication No. 2002-135077 is designed to suppress transverse mode spurious responses in a surface acoustic wave device that includes a quartz substrate, but not a SiO2 film, and an IDT electrode made of aluminum on the substrate. Thus, a way to suppress spurious responses whose exact cause is unknown and which occur when a SiO2 protective film is provided on a surface acoustic wave device having an aluminum IDT electrode as described in Japanese Unexamined Patent Application Publication No. 2006-165746 has not previously been known or disclosed.